Half-Heuslers (HHs) are intermetallic compounds which have great potential as high temperature thermoelectric materials for power generation. However, the dimensionless thermoelectric figure-of-merit (ZT) of HHs is lower than that of the most state-of-the-art thermoelectric materials. HHs are complex compounds: MCoSb (p-type) and MNiSn (n-type), where M can be Ti or Zr or Hf or combination of two or three of the elements. They form in cubic crystal structure with a F4/3m (No. 216) space group. These phases are semiconductors with 18 valence electron count (VEC) per unit cell and a narrow energy gap. The Fermi level is slightly above the top of the valence band. The HH phases have a fairly decent Seebeck coefficient with moderate electrical conductivity. The performance of thermoelectric materials depends on ZT, defined by ZT=(S2σ/κ)T, where σ is the electrical conductivity, S the Seebeck coefficient, κ the thermal conductivity, and T the absolute temperature. Half-Heusler compounds may be good thermoelectric materials due to their high power factor (S2σ). It has been reported that the MNiSn phases are promising n-type thermoelectric materials with exceptionally large power factors and MCoSb phases are promising p-type materials. In recent years, different approaches have been reported that have improved the ZT of half-Heusler compounds by mainly optimizing the compositions. However, the observed peak ZT is only around 0.5 for p-type and 0.8 for n-type due to their relatively high thermal conductivity.